I think you should have gotten a 12" or even 2 12's instead of one 15".
I also have an intrepid, My box houses 2 powerbass m12's, Before displacements its 4.8^cu. ft. internal volume, and about 4.2 after displacements, It is as high as the rear 6x9's well let me have it, and 21 inches deep and...